1. Field of the Invention
This invention relates to a printing method, a print apparatus, a program, and a storage medium, and more specifically, to those that are applied to position adjustment of ink dot formation in an inkjet printing system with a preferable result. In addition, this invention is applicable to a copier, a facsimile having a communication system, equipment having a printing portion, such as a word processor, and further an industrial printing system that is combined with any of various kinds of processing devices in a sophisticated manner as well as general print units.
2. Description of the Related Art
A so-called serial scan type image printing system that executes a printing operation with a printing head that is a major constituent of a printing portion being scanned on a printing medium finds applications in various image formation. Especially, those that use the inkjet system are becoming rapidly popular because those printers support high resolution and color printing and consequently image quality has made remarkable improvement in recent years. The equipment of this kind uses the so-called multi-nozzle head formed by accumulating and disposing ejection openings for ejecting ink, for example, as a drop. At present, it has become possible to form images having higher resolution with this multi-nozzle head by increasing integration density of the ejection openings and lessening the amount of ink discharge per dot. On the other hand, in order to realize the image quality comparable to that attained with that of the silver salt photograph, there are being developed a variety of technologies, such as one whereby, in addition to 4 colors of inks (inks of following colors: cyan, magenta, yellow, and black) that are basic colors, inks of light colors obtained by decreasing these concentrations are used simultaneously. In addition, regarding lowering of the printing speed that is anticipated with increasing quality of printing, the capability of resolving it has been realized by adopting such technologies as an increment in the number of printing elements, improvement of driving frequency, and even bi-directional printing, and hence good throughput is being attained increasingly.
FIG. 15 schematically shows a general construction of a printer for conducting printing using the above-mentioned multi-nozzle head. In this figure, reference numeral 1901 denotes a head cartridge provided in the printing system so as to support one of 4 colors of inks, for example, black (K), cyan (C), magenta (M), and yellow (Y). Each head cartridge 1901 is composed of an ink tank 1902T in which an ink of one of those colors is filled and a head portion 1902I formed by arranging a large number of ejection openings each capable of ejecting the ink supplied from the tank on the printing medium.
Reference numeral 1903 denotes a paper feed roller (feed roller) that rotates in a direction indicated by an arrow in the figure to convey a printing medium (printing paper) 1907 in a y-direction (sub scanning direction) at any time while sandwiching the printing medium 1907 in cooperation with an auxiliary roller 1904. Moreover, reference numeral 1905 denotes a pair of feed rollers for supplying the printing paper 1907 to a printing position while sandwiching it and also performing a function of holding the printing paper 1907 flatly between the rollers 1903 and 1904.
Reference numeral 1906 denotes a carriage that supports four head cartridges 1901 and moves these in a main scanning direction in the printing operation. This carriage 1906 is moved to a position (home position) h shown with the dashed line in the figure when printing is not being executed or when a recovery operation for keeping ink ejecting performance of the head portion 1902H excellent is conducted.
The carriage 1906 that has been moved to the home position h before start of printing starts to be moved in an x-direction upon receipt of a print start command. Then, the ink is ejected from a plurality (n units) of ejection openings provided on the head portion 1902H according to print data, whereby printing for a width corresponding to a range of an ejection opening arrangement is performed. Thus, when the printing operation is completed to an end of the printing paper 1907 in the x-direction, in the case of single-direction printing, the carriage 1906 returns the home position h and performs a printing operation again in the x-direction, whereas in the case of bi-directional printing, the printing operation is also performed when the carriage 1916 moves in the minus x-direction toward the home position h. In either case, before the next printing operation is started after the completion of one printing operation (1 scan) in one direction, the printing paper 1907 is fed by a predetermined amount (equal to the width of the ejection opening arrangement) by the paper feeding roller 1903 revolving in an arrow direction in the figure by a predetermined amount. In this way, the printing operation of one scan and the feeding of the printing paper by the predetermined width are repeated, and thereby the printing of data for one sheet of the printing paper is completed.
In such a serial-type inkjet printer, in order to support image printing with higher resolution, various contrivances have been adopted regarding the construction of the head portion and the printing method.
For example, due to a constraint in manufacture of the multi-nozzle head, there is inevitably a limit in the density of a nozzle array on a single line.
FIG. 16A shows an example of the head for realizing still higher-density printing to circumvent the problem. In this head, a large number of ejection openings are arranged on a line in a y-direction with a predetermined pitch py, and this ejection opening array is also provided on another line that is displaced by a predetermined distance px, making two ejection opening arrays in the x-direction, wherein the ejection opening array on the other line is shifted to those on the one line in the y-direction by py/2. By this arrangement, a resolution twice as high as the resolution of a single array of ejection openings is realized. Furthermore, when applying this contrivance to the apparatus in FIG. 15, it is possible to arrange the heads, each of which is for one color as shown in FIG. 16A, side by side in the x-direction to support 6 colors of inks. In this construction, only with proper adjustment of ejecting timing for both arrays of ejection openings, color printing with a resolution twice as high as the resolution attained with a single array of ejection openings can be achieved.
Incidentally, there is also a technology, as disclosed in U.S. Pat. No. 4,920,355 and Japanese Patent Application Laid-open No.7-242025 (1995), in which the amount of paper feeding for each scanning of printing is set to a predetermined number of pixels smaller than the width of the nozzle array while the construction of a multi-nozzle array is kept to a low resolution, so that high-resolution printing is achieved. Such print methods are called the interlace printing method hereinafter.
With reference to FIG. 17, this interlace printing method will be described briefly. Here, printing is assumed such that a head H in which the ejection openings are arranged by a pitch of 300 dpi (dot per inch) is used to complete an image of 1200 dpi. For simplicity, the number of ejection openings is set to nine, and the amount of paper feeding to be done at each scanning of printing is set equal to nine pixels for 1200 dpi. Rasters printed in the forward travel are represented with solid lines, the rasters printed in the reverse travel are represented with dashed lines, and the figure shows these rasters being formed alternately.
Here, an example where a paper is fed by a constant amount, i.e., nine pixels for each feeding, is described, but the interlace printing is not restricted to this construction. It can be said that any construction in which an image having a pitch finer than the original pitch of the arrangement of the ejection openings is completed by a plurality of printing scanning lines belongs to the interlace printing method even if the amount of paper feeding is not always constant. In any case, the interlace printing method enables image printing with higher resolution than the original resolution corresponding to the array of ejection openings.
By various methods described in the foregoing, printing of images with resolution higher than that of the nozzle array is made possible.
On the other hand, the printing resolution of the printing system is not necessarily equal to the input resolution from a host device serving as an image data supply source, and printing systems of recent years are capable of printing according to plural input resolutions. For example, when an output of high-definition monochromatic characters and patterns is desired, it is preferable to print binary images with the same input resolution as the highest resolution of the printing system. When high-speed processing is desired or when it is desired for a load on host equipment to be lessened, if a printer with a printing resolution of 2400 dpi is enabled to receive input of image data with a quarter of its resolution, namely 600 dpi, it is possible to shorten a transfer time of data from the host device. At this time, since one output pixel represents a binary value, one input pixel having a multivalued level can be printed in a gradation representation of 17 values by output pixels of 4×4. Such an approach has already been proposed and put into practical use.
As one example, a technology disclosed in Japanese Patent Application Laid-open No 9-046522 (1997) will be described. If the input resolution is 300 dpi and the output resolution of a printer is 600 dpi, the printer can represent 5-valued gradation by a dot arrangement of 2×2. Denoting 5-valued levels as “level 0” though “level 4”, each one level of gradation can be represented by a plurality of dot patterns (patterns of dot arrangement) except “level 0” and “level 4,” as shown in FIG. 18. Japanese Patent Application Laid-open No. 9-046522 (1997) discloses a method whereby the plurality of patterns are arranged sequentially or at random. With this arrangement of the plurality of patterns, a dot arrangement constituting a pixel at each level of gradation is not fixed; therefore this method has an effect of reducing the so-called “sweep-together phenomenon” that may appear at pseudo outlines and edges of an image when the pseudo half tone processing is performed, etc. Moreover, this method has also an effect of averaging use states of nozzles in the printing head.
Moreover, in the case where such a head as shown in FIG. 16A is used, since even-number rasters and odd-number rasters that are to be arranged alternately in the y-direction (sub scanning direction) are printed by different ejection opening arrays, impact positions of dots formed by one ejection opening array are displaced slightly from those formed by other array and hence there may occur deterioration in the image quality. One of the causes of this is a phenomenon that a plane of the head (face plane) on which the ejection openings are provided is deformed by swelling of the ink, temperature increase, etc. For example, if a convex deformation has occurred therein between the ejection opening array taking part in printing of the odd-number rasters (odd-number nozzle array) and the ejection opening array taking part in even-number rasters (even-number nozzle array), the ink is ejected from each ejection opening array in a different direction, i.e., in the shape of a character “Λ.” Displacement of ink impact position between rasters arising from such a phenomenon causes detrimental effects on the image quality, although the amount is very little, and the deterioration becomes significant in high-resolution images obtained by a binarization method, such as the error diffusion method.
That technology has been put into practice wherein, in the case where a plurality of dot patterns are used sequentially for one level of gradation, four kinds of dot patterns are arranged in the main scanning direction with a fixed order as a minimum unit and the unit is used repeatedly for every four input pixels of an image.
Such a printing method is effective, especially, in high-resolution printing systems. For example, in a printer that is intended to realize photographic image quality, input resolution equal to or better than visual resolution is not necessary: if a resolution of about 600 dpi is obtained, it is effective to enhance tone (correctness in gradation) in addition to its attainment. Moreover, in the case where there occurs ink impact position displacement between rasters arising from the phenomenon as shown in FIG. 16B, the deterioration in the image quality can be controlled consequently by using a plurality of patterns for one level of gradation sequentially.
However, dot patterns affect the image quality largely, and in the case where plural kinds of patterns of dot arrangements each displaying the same level of gradation are arranged at random in the main scanning direction, there is the possibility that noises such as roughness and a sense of granularity may occur in the image. In order to prevent occurrence of these noises, it is necessary to use such a dot arrangement that is suited for the resolution and a dot size.
Although designing an arrangement of dot patterns in the main scanning direction in a large area makes it easy to control a spatial frequency characteristic of output images, it will use much capacity of memory, etc., because of a large arrangement matrix. In the case where the area of arrangement of dot patterns in the main scanning direction cannot be secured wide enough because of a constraint of memory capacity, a period of the area appears in the image, gives a sense of noise, and leads to deterioration in the image quality.
On the other hand, in the case where a plurality of dot patterns are arranged sequentially in the main scanning direction, a pattern arising from periodicity of the arranged dot patterns, such as a period of dot patterns repeated in the main scanning direction and a period thereof repeated in the sub scanning direction, may appear as noises, such as a streak, in the image.
FIG. 19 is an explanatory diagram of a printing result in the case where a matrix (4×1), in which four kinds of dot patterns for “level 2” are arranged, is created as a minimum unit and the matrix is used repeatedly in the main scanning direction (the horizontal direction in the figure). FIG. 20 is an explanatory diagram of a printing result in the case where a matrix (1×4), in which four kinds of dot patterns for “level 2” are arranged, is created as a minimum unit and the matrix is used repeatedly in a sub scanning direction (the vertical direction in the figure).
In this way, in the case where a matrix (A×1 or 1×A) having a fixed arrangement of a plurality of dot patterns in the main scanning direction or in the sub scanning direction (denoting an arrangement of A) is created as a minimum unit and the matrix is used repeatedly so as to correspond to the input pixel of the image, the matrix will be arranged in the sub scanning direction and in the main scanning direction. As a result, as shown in FIG. 19 and FIG. 20, the same dot patterns lie in a row in the sub scanning direction and in the main scanning direction, and a vertical streak and a horizontal streak in the arrangement directions may appear in the image to deteriorate the image quality.